1. Field of the Invention
The invention relates to an improvement in lockable gelatine capsules and a method and apparatus for roller shaping a locking groove in the forming pin for the cap.
2. Description of the Prior Art
Self locking gelatine capsules are widely used in the pharmaceutical industry. A common type is described in U.S. Pat. No. 3,399,803 entitled "Self-Locking Medicament Capsule" and assigned to Parke Davis and Company, Detroit, Mich. A corresponding Canadian Pat. No. 805,125 entitled "Self-Locking Gelatine Capsule" issued on Jan. 28, 1969. Both patents describe a gelatine capsule cap having a circumferential annular beveled ridge which extends inward from the cap sidewall. Optionally, the capsule body may have a complimentary circumferential groove to mate with the inwardly projecting circumferential ridge on the cap body. The inwardly directed circumferential ridge on the gelatine cap friction locks with the gelatine body or mates with the circumferential grooves in the gelatine body of the capsule, if such a mating groove is present on the capsule body. The beveled ridge on the capsule cap has a triangular contour including leading and trailing sidewall faces having a bevel angle up to about 10.degree. and an optional flat surface between the two beveled sidewalls.
Similar capsule structures are described in U.S. Pat. Nos. 3,508,678 and 3,664,495. Canadian Pat. No. 930,674 issued on July 24, 1973 and entitled "Locking Capsule" is the foreign counterpart to U.S. Pat. No. 3,664,495. U.S. Pat. Nos. 3,508,678 and 3,664,495 disclose alternative embodiments for self-locking capsules having locking grooves and ridges in the body and caps of the capsule respectively. In addition, both patents further describe the use of indents to provide additional locking security. The indents, known as "prelocks", provide a mechanical fit as distinguished from the friction fit between the inwardly facing locking ridge of the capsule cap and the optional locking groove of the capsule body. Pre-locking indents are now a common feature of modern gelatine capsules.
Additional efforts have been made to improve the locking characteristics of self-locking gelatine capsules. For example, U.S. Pat. No. 3,584,759 entitled "Separation-Resistant Capsule" describes a gelatine capsule in which the cap and body portions each increase in diameter from their domed end towards their open-end so that they mate tightly with each other. The capsule includes a sealing zone where the actual mating of the cap and body takes place. U.S. Pat. No. 4,247,006 describes another technique in which the open end of the body portion of the capsule has a slightly reduced diameter at its open end so as to improve its mating characteristics with respect to the cap portion of the capsule.
The prior art described above tends to have a number of disadvantages. First, the locking ridge and complimentary groove structure of the capsules tends to have angular, i.e. sharp profiles. Note, for example, the "triangular" contour of the locking ridge described in U.S. Pat. No. 3,399,803. The sharp angle of the locking ridge defines a smaller area of locking contact and therefore a weaker frictional mate with the capsule body. Second, the angular nature of the locking ridge also tends to weaken the sidewall of the gelatine cap. Therefore, it is not uncommon to find that a substantial number of capsules are ultimately broken in the cap region due to the relative weakness in the vicinity of the locking ridge. Third, prior art forming pins tend to wear out quickly. In contrast, the capsule pins formed by the method described in this disclosure tend to have a significantly longer life.